Under certain local load conditions, a grid-tie inverter in a distributed generation (DG) system can maintain sustainable output power at a sustainable voltage and frequency when it is suddenly disconnected from the main grid, creating an islanding condition. Operating the inverter in an islanding condition can pose a safety hazard to service personnel and cause damage to the power system. Both the Institute of Electrical and Electronics Engineers (IEEE) Standard 1547 and Underwriters Laboratories (UL) 1741 require the inverter to be shut down within two seconds once it is islanded from the main grid.
There are generally two types of anti-islanding techniques: passive and active. Passive techniques use information available at the inverter side to detect islanding while active techniques introduce external perturbations at the output of the inverter to detect and trip islanding.
An existing active technique involves using a phase shift algorithm to move the inverter's frequency into the Over Frequency Protection/Under Frequency Protection (OFP/UFP) trip window. The phase shift algorithm can repeatedly introduce phase shifts in sequential cycles of alternating current (AC), such as 0 degrees for 30 AC cycles, +6 degrees in one AC cycle, +12 degrees in one AC cycle, +6 degrees in one AC cycle, 0 degrees in one AC cycle, −6 degrees in one AC cycle, −12 degrees in one AC cycle, −6 degrees in one AC cycle. These phase shifts can be as large as 12 degrees and have an adverse impact on the inverter's output power and voltage at the point of common coupling (PCC).
Another existing active technique modifies the phase shift loop into a more aggressive loop to meet the anti-islanding requirements per IEEE Standard 1547, such as 0 degrees in 30 AC cycles, +6 degrees in two AC cycles, +12 degrees in two AC cycles, +6 degrees in two AC cycles, 0 degrees in one AC cycle, −6 degrees in two AC cycles, −12 degrees in two AC cycles, and −6 degrees in two AC cycles. An adverse consequence of this modified technique is that even higher power and voltage swing are introduced to the inverter's output, further downgrading the power quality and efficiency of the inverter.